In a typical TDM system, a transmitter samples pulse signals of relatively low pulse repetition frequency from various data sources or channels and interleaves them with one another to form an aggregate data stream that is transmitted by a high speed aggregate channel to a remote receiver. Ordinarily, the TDM transmitter inserts the signals representative of a single bit or a single character in a single time slot in the aggregate data stream and interleaves the signals from different channels on a bit-by-bit or character-by-character basis so that adjacent time slots contain signals from different channels. However, different size blocks of signals can be used if desired. At the receiver, the individual bits or characters are separated from one another and allocated to various low frequency data channels similar to those at the transmitter.
To permit proper decoding of the data stream at the receiver, the transmitter interleaves the signals from the various data channels in accordance with a fixed schedule which it repeats endlessly and the receiver uses the same schedule to decode the data stream. Each cycle of the schedule is called a frame or an aggregate frame. In addition to data signals, each frame ordinarily includes synchronization signals called frame sync words and various control signals, both for individual channels and for the entire TDM system. Typically, the synchronization and control signals take up a small portion (less than 5%) of the total frame which is referred to as the overhead. To simplify the generation of the signals used to select the particular data channel from which a bit or character is to be transmitted, it is customery to sample the data channels in a fixed pattern which is repeated numerous times within each frame. Each such cycle of repetition is called a subframe.
While the synchronization and control signals take up a relatively small portion of the aggregate frame, transmission of these signals causes numerous problems in the efficient operation of the TDM. One such problem is the competition between the need to transmit promptly changes in the control signals for a channel and the need to ensure that each channel's control signals are transmitted periodically so that the system may recover if there is a communication failure. The protocol used in many TDMs is to transmit a change in the control signals for a channel in the next available time slot after a change has occurred. If a time slot becomes available for transmission and there are no changes to be transmitted, a counter is incremented and the incremented value is used to select a channel whose control signals are transmitted. This, however, can lead to serious problems if one or more channels have enough control activity to occupy all of the available time slots because the TDM will never transmit the controls for the remaining channels. As a result, if there is a failure in communications, it may be difficult, if not impossible, to reestablish communications automatically.